Measuring the leak rate of a fluid in a sealed container is a very important procedure in many different applications. One can measure the leak rate in a pressurized gas bottle to determine whether or not the bottle exhibits a leak after a pressurized gas is placed in the bottle. As one can ascertain, for a fixed volume container one can measure the leak rate by measuring the pressure and temperature inside the container. By using the ideal gas law, which is PV=nRT (or for higher pressure the Van der Waals equation), it is easy to see that taking the ratio of pressure to temperature P/T gives a measure of the molar density (n/V) which only changes with a leak in the system. This is a well known principle and the assignee herein, namely Kulite Semiconductor Products, Inc. has a number of patents and pending applications concerning the use of the above-noted gas law to determine leaks in a container. See for example U.S. Pat. No. 5,428,985 entitled “Gas Leak Detection Apparatus and Methods” issued on Jul. 4, 1995 to A. D. Kurtz et al and assigned to the assignee herein. See also U.S. Pat. No. 4,766,763 entitled “Gas Leak Detection Apparatus and Methods” issued on Aug. 30, 1988 to A. D. Kurtz the inventor herein, and also assigned to the assignee herein. The above-noted patents basically use a pressure transducer which produces an output voltage proportional to the gas pressure inside a container or a vessel to be monitored. The above-noted patents employ amplifier circuits and operate on the principle of the ideal gas law which states that at normal temperatures and pressures the pressure of a gas is given by the equation PV=nRT, where V is volume occupied by the gas, P is the pressure of the gas, R is the universal gas constant, n is the number of moles of gas within the volume, and T is the absolute temperature of the gas. The output of the apparatus is proportional to the ratio of pressure to temperature, namely P/T and accordingly is independent of temperature induced pressure changes. Thus for a fixed volume a change in the value of the output signal would be indicative of the change in the number of moles of gas contained in the vessel, thereby denoting or determining a leak condition of the container. The pressure transducers may employ deflectable diaphragms which operate in conjunction with piezoresistors which typically are arranged in a Wheatstone bridge array. Such pressure transducers are well known, and Kulite Semiconductor Products, Inc., the assignee herein, has many patents which involve fabrication and implementation of pressure transducers including those capable of operating at high pressures in high temperature environments. In regard to the above, a much more complicated situation occurs when the volume (V) of the container changes over time. In general, to measure a gas leak in this case the volume must also be measured. One specific application which can create major problems is the detection of leaks in a shock absorber, such as those used in aircraft or in automobiles. While a leak in an automobile shock absorber is typically not a life threatening event, a leak in a shock absorber or a landing gear of an airplane can be catastrophic and should be quickly identified.
There is disclosed apparatus and a method for measuring leaks in an aircraft shock absorber or other similar device by measuring the pressure, temperature and displacement.